What you are saying would be even MORE true for the enrollment counts. The same student identifier could turn up possibly multiple times IN a department AND ALSO ACROSS DEPARTMENTS.

With COUNT(<identifier>), the percentage calculation seems to be work on each number in the row, being divided by the sum of the numbers in the row. The numbers displayed are treated just as I would expect--as in a spreadsheet.

Why would unduplicating the counts cause the sum to be exactly halved (or the individual numbers to be doubled)?

So yes I see the problem, what I had said before is true in your case:

As you can see, Pidm 711 occurs in multiple different C Div and multiple Terms. However your % of total will only count this Pidm as ONE because it's doing a count distinct of Pidm of the whole dataset.

So what you need to do is use an LOD calculation, I have used fixed and created this:

This means for every term and C Div, it is calculating the count distinct of Pidm. Then at an overall level, it is adding it all up.

So the final view, I use this new field and I get is this:

Thanks and please see the attached workbook with this example. The viewsI have created have an 'ML' at the end of the worksheet names.